Stability of sevoflurane (fluoromethyl 2,2,2-trifluoro-1-(trifluoromethyl)ethyl ether), a new inhalational anesthetic, in sodalime was examined, and the products of their reaction were identified and quantitated. Five reaction products were identified: fluoromethyl 2,2-difluoro-1-(trifluoromethyl)vinyl ether, a dehydrofluorination product of sevoflurane which is contained as an impurity in sevoflurane preparations, fluoromethyl 2-methoxy-2,2-difluoro-1-(trifluoromethyl)ethyl ether, a methylation product of fluoromethyl 2,2-difluoro-1-(trifluoromethyl)vinyl ether and/or sevoflurane, fluoromethyl 2-methoxy-2,2-difluoro-1-(difluoromethylene)ethyl ether and two isomers of fluoromethyl 2-methoxy-2-fluoro-1-(trifluoromethyl)vinyl ether, dehydrofluorination products of fluoromethyl 2-methoxy-2,2-difluoro-1-(trifluoromethyl)ethyl ether. In a closed anesthetic circuit with sodalime connected to a model lung, fluoromethyl 2,2-difluoro-1-(trifluoromethyl)vinyl ether increased and reached a plateau. Fluoromethyl 2-methoxy-2,2-difluoro-1-(trifluoromethyl)ethyl ether increased linearly and other substances were detected only in a trace amount. In the semi-closed anesthetic circuit with sodalime supplied with 6 liters/min fresh gas flow, no reaction products were detected except fluoromethyl 2,2-difluoro-1- (trifluoromethyl)vinyl ether, which showed a maximum concentration of 2 and 4 ppm when the feeding concentration of sevoflurane was 1.7 and 2.7%, respectively. It is known that fluoromethyl 2,2-difluoro-1-(trifluoromethyl)vinyl ether is a weak anesthetic with AC_50 =3.58%, LC_50 =10.17% and AI=2.84. These results indicate that sevoflurane can be used with sodalime in a semi-closed anesthetic circuit. This study was supported in part by a Grant-in-aid for Science Research from the Ministry of Education, Science and Culture of Japan and a Grant-in-aid from the Association for the Advancement of Medicine of the Tsuchiya Foundation.